Vacuum fluorescent (VF) displays require a filament power supply to heat the filament to a temperature suitable for proper emission of electrons which are accelerated by an anode potential onto a fluorescent material to emit light. In VF displays which are physically short (up to about three inches) a DC filament voltage of typically less than three volts can be used. The gradient in filament to anode voltage over the length of the tube can be compensated for by physically varying the filament to anode spacing over the length of the tube. In this case, a simple dropping resistor from the power source (usually a 12 V battery in automotive applications) can be used. However, variations in battery voltage will cause variations in the filament voltage and thus light intensity.
In longer VF displays where higher filament voltages are required, an AC filament must be employed, so that the filament to anode voltage gradient reverses periodically (and rapidly enough to avoid producing any apparent flicker), resulting in a more uniform intensity over the length of the VF tube. In simple displays which can use battery voltage for the anode power supply, a sine-wave filament supply is typically used. Usually a transformer is used with a resonant primary circuit and a push-pull transistor drive arrangement. The resulting sine-wave output voltage (typically at 30 kHz) is proportional to the battery source voltage. A filament synch signal is required when the display is dimmed using pulse width modulation, insuring that the dimming pulse is actuated on opposite phases of the filament waveform to eliminate beating between the filament and dimming frequencies which would result in flickering of the display.
In more complicated displays using higher-than-battery voltages for the grid and anode supplies, generally a switching flyback power supply circuit is employed. This also requires a transformer, but can provide multiple output voltages. One of the regulated DC outputs can be used to power a sine-wave filament supply as described above, but this requires an additional transformer. Alternatively, an additional winding can be added to the flyback transformer to provide the filament voltage directly. This approach, however, yields a very non-sinusoidal output waveform. At switching frequencies of 50 kHz or greater, the harmonic content is significant, especially in the AM broadcast band, resulting in objectionable radio frequency interference (RFI). Filtering of the output waveform is possible, but adds extra components and cost. Also, when the filament power supply is powered from the output of the switching power supply, all the power for the filament in addition to the transformer, must be converted through the switching transistor and transformer, dissipating significant amounts of power and usually requiring a heat sink and larger transformer, capacitors and diodes. A filament synchronization signal is typically derived from the filament AC power supply and connected to the circuit controlling the pulse width modulated (PWM) dimming of the display.
The use of transformers requires some hand assembly of components during fabrication and result in large, relatively heavy power supplies. Due to power conversion inefficiencies, heat generation results in the need for heat sinks.
Thus in existing systems which use battery voltage for the anodes and grids of VF displays, the filament power supply is generally a sine-wave type. The operating frequency is typically about 30 kHz to reduce the size of the transformer and resonating capacitor. This supply is basically unregulated when its source voltage is directly from the battery. A filament synchronization signal is required to reduce flicker at low duty cycle dimming values. The center-tapped output winding of this configuration can easily be referenced to ground, or biased above ground to provide a higher voltage to facilitate cutoff, if required for the VF display tube. In systems which use a switching power supply to provide higher voltages for anode voltage, a separate filament transformer is excited from the secondary of the switching power supply. Sine wave filament power sources also yield a high frequency voltage with harmonics which result in RF interference, especially in the AM band.